This invention relates to composite polymeric materials, particularly composite materials containing rod-like aromatic heterocyclic polymers dispersed in a polymeric matrix.
Chopped fiber reinforced plastics are currently being used in the fabrication of a wide variety of components. There are several disadvantages in the use of fiber for reinforcement of plastic. In the case of chopped glass fibers, a large amount of fiber, generally a minimum of 30 percent by weight, is necessary for reinforcement because of the low reinforcing effect of the fiber. There is a practical processing limit on the effective fiber length. A macroscopically long fiber length is required with due regard to breaking or destruction of the fiber during processing, particularly molding. Composite materials containing chopped fibers are generally less processable than their non-reinforced counterparts. The shape of moldings is often limited to simple block or sheet forms. Films or filaments cannot be formed from chopped glass fiber-reinforced plastics. Other disadvantages of these materials include poor surface properties of molded articles, an anisotropy in dynamic properties of moldings, molding defects due to heterogeniety of the polymeric materials, and low cycle time in processing.
A need exists for high strength reinforced composites and a method for their manufacture which possess at least the following desirable prerequisites: (1) non-reliance on fiber reinforcement for the attainment of high strength properties; (2) circumvention of the complexities of current composite fabrication procedures, and (3) elimimation of any possibility of fiber-polymer interface problems.
Various attempts have been made to overcome some of the above-described disadvantages of chopped-fiber-reinforced plastics. One approach described by Helminiak et al, U.S. Pat. Nos. 4,207,407 and 4,377,546, comprises the dispersion of an intrinsically rigid rod-like heterocyclic polymer in a flexible, coil-like heterocyclic polymer. These composites are referred to as molecular composites. While this approach represents a valuable contribution to the art, it has certain drawbacks. For example, poly (p-phenylene benzobisthiazole) (PBT) has superior mechanical properties and thermal stability. However, PBT degrades before it melts; therefore, processing of a composite containing PBT must be carried out in a solution state with an acid, such as methanesulfonic-acid (MSA), as the solvent. Relatively few flexible coil polymers can be dissolved in or are stable in MSA, thus limiting the choice of matrix polymers. Molecular composites based on PBT and poly-2,5-benzimidazole (ABPBI) have been fabricated into fibers and thin films. However, ABPBI does not have a glass transition (Tg) temperature. Therefore, molecular composites containing ABPBI are difficult to thermally consolidate into thicker specimens. To overcome this problem, thermoplastic matrices have been used so that the molecular composite films could be laminated. However, thicker specimens fabricated using thermoplastic matrices are limited to use at temperatures below the Tg of the matrix polymer(s). Conventional thermoset resins, such as bismaleimides, epoxies and the like, are not stable in the acid medium used to process the rigid-rod polymer, and cannot be used as host matrices for molecular composites.
Accordingly, it is an object of the present invention to provide a molecular composite having thermoplastic and thermosetting properties.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art.